Magnetoresistive Random Access Memory (MRAM) is a non-volatile memory technology that uses magnetic elements. For example, Spin Transfer Torque Magnetoresistive Random Access Memory (STT-MRAM) uses electrons that become spin-polarized as the electrons pass through a thin film (spin filter). STT-MRAM is also known as Spin Transfer Torque RAM (STT-RAM), Spin Torque Transfer Magnetization Switching RAM (Spin-RAM), and Spin Momentum Transfer (SMT-RAM).
FIG. 1 illustrates a conventional STT-MRAM bit cell 100. The STT-MRAM bit cell 100 includes magnetic tunnel junction (MTJ) storage element 105, a transistor 101, a bit line 102 and a word line 103. The MTJ storage element is formed, for example, from at least two ferromagnetic layers (a pinned layer and a free layer), each of which can hold a magnetic field or polarization, separated by a thin non-magnetic insulating layer (tunneling barrier). Electrons from the two ferromagnetic layers can penetrate through the tunneling barrier due to a tunneling effect under a bias voltage applied to the ferromagnetic layers. The magnetic polarization of the free layer can be reversed so that the polarity of the pinned layer and the free layer are either substantially aligned or opposite. The resistance of the electrical path through the MTJ will vary depending on the alignment of the polarizations of the pinned and free layers. This variance in resistance can be used to program and read the bit cell 100. The STT-MRAM bit cell 100 also includes a source line 104, a sense amplifier 108, read/write circuitry 106 and a bit line reference 107. Those skilled in the art will appreciate the operation and construction of the memory cell 100 is provided merely as an example.
With reference to FIGS. 2A-C, conventional MTJ storage elements generally are formed by first patterning a bottom fixed layer, forming a single damascene, depositing the tunneling barrier/free layer/top electrode stack, and performing a chemical mechanical polishing (CMP) step. Additional details are provided, for example, in M. Hosomi, et al., A Novel Nonvolatile Memory with Spin Transfer Torque Magnetoresistive Magnetization Switching Spin-RAM, proceedings of IEDM conference (2005), which is incorporated herein by reference in its entirety.
For example, as shown in FIG. 3, conventional MTJ storage elements generally are formed on a bottom electrode 302 such as a Si substrate. One or more seed layers (not shown) may be formed on the bottom electrode 302. An antiferromagnetic (AFM) layer 304 is first formed on the bottom electrode 302, and then a first ferromagnetic layer is formed on top of the AFM layer. The first ferromagnetic layer is “pinned” with a fixed magnetization to form a pinned layer. The pinned layer may include one or more layers, such as a bottom pinned layer 306, a coupling layer 308 typically formed of a non-magnetic metal such as ruthenium, and a top pinned layer 310. A tunneling barrier layer 312 is formed of an insulator such as a metal oxide on top of the pinned layer. A free layer 314 is formed of a second ferromagnetic layer directly on top of the tunneling barrier 312. A top electrode or hardmask layer 316 (e.g., tantalum) is formed on top of the free layer 314.
Next, the MTJ stack 300 is subjected to a magnetic annealing process in a vacuum. A pattern is then applied to the MTJ stack using a lithography technique. A photoresist (not shown in FIG. 3) is formed on top of the hardmask layer 316. The patterned cell size may be larger than the final size. Each of the aforementioned layers can be comprised of one or more layers or films.
Next, the MTJ stack 300 is etched using an etching process such as reactive ion etching. The etching process includes trimming the size of the photoresist, patterning the hardmask 316, removing the photoresist, etching the free layer 314, etching the barrier layer 312, etching the pinned layers 306, 308 and 310, and etching the pinning layer AFM 304. Next, a passivation layer is deposited to protect the MTJ storage element and the interlayer dielectric (ILD) insulator layer 318. A combination stack may be needed, along with a low deposition temperature to protect the MTJ and promote adhesion between the MTJ and ILD. Finally, planarization and metallization is performed.
The MTJ stack 300 is susceptible to damage during the etching process due to redeposition of etching byproducts. The step involving removal of photoresist may include processes such as oxygen ashing. Oxygen ashing can cause damage to the hardmask layer 316 during the photoresist removal process. Oxygen ashing can also cause damage to upper portions 320 of sidewalls of the free layer 314. As described above, the etching process proceeds from etching the hardmask layer 316 at the top of the MTJ stack 300 towards etching the pinned layers at the bottom of the stack. As the etching process progresses deeper down the MTJ stack, damage can be caused to sidewalls 322 of the free layer 314. As the etching process proceeds further down the stack, the upper portions 324 and lower portions 326 of the sidewalls of the barrier layer 312 may also be impacted.
As some of the etching byproducts may be conductive, damages to the sidewalls of the MTJ due to redeposition of such etching byproducts may lead to leakage paths, thereby reducing the magnetic resistance (MR) ratio of the MTJ. Such process related damages may result in significantly lower yields. There is a need for techniques which protect the MTJs from damages caused during the fabrication process.